Method and apparatus for reducing metal bodies



Oct. 26, 1943. E- oR e 2,332,803

METHOD AND APPARATUS FOR REDUCING METAL BODIES Filed Dec. 2 1941 3Sheets-Sheet 1 v lywemr:

Oct. 26, 1943. E. "r. LORIG 2,

METHOD AND APPARATUS FOR REDUCING METAL BODIES Filed Dec. 20, 1941 3Sheets-Sheet 2 FIG. 2.

F' IE. L3.

Oct; 26, 1943. Y E. T. LORIG 2,

METHOD AND APPARATUS FOR REDUCiNG METAL BODIES Filed Dec. 20 1941 aSheets-Sheet s aw/v 7. LOB/6,

Patented Oct. 26, 1943 Mnrnon AND APPARATUS FOR REDUCING METAL nonrrzsEdwin,'l. Lorig, Dormont, Pa., asslgnor to Carnegie-Illinois SteelCorporation, a corporation of New Jersey Application December 2c, 1941,Serial No. 423,848

13 Claims.

This invention relates to the reduction of the cross-sectional area ofmetal bodies by-plastic deformation. Although not limited thereto. it ispeculiarly well-suited to the tensionrolling of metals of low elasticlimit, such as steel strips, sheets and like plastically deformablemetals of any description.

There are two fundamental forces, which, when applied in ,suflicientmagnitude either singly or in combination to a metal body, cause it toflow plastically to alter and reduce its cross-sectional area and shape.These are the forces'of compression and tension. The former is usuallyapplied to a work piece by forcing it to move between rolls or dieshaving a restricted area of pass, which compresses the work piece into asmaller section and displaces its volume longitudinally. Tension may beapplied alone to cause the metal to stretch, thus to become longer andof lesser cross-sectional area, or in combination with compression, asis the case when non-driven rolls or dies are employed to compress thework piece, tension must be utilized to draw it through.

Compression and tension may coact to elongate and reduce thecross-sectional'area of a work piece, and this coaction may take placein' several ways:

Firstly, as has already ben mentioned, a metal body may be pulledthrough stationary or roiler 3o dies, whereby the'metal at the exit sideof the zone of compression is under tension, while that behind the zoneof compression is not under tension.- A

Secondly, the compression dies'may be driven to cause the metal to movetherethrough and become'reduced, which movement may be resistedanteriorly of the zone of compression to impose tension on the materialapproaching the rolls, in the absence of thereof.

Thirdly, a work piece may be tensioned before and after the compression--zone,, so that,-5with driven rolls, the values offorward and backtension may be different, or of the same order, as

where a work piece is tensioned through nondriven dies.

tension at the exit sidesuch ,bodies have a thickness to widthfactor"-approaching unity, or are round, as inithe. case of bars, rods,and wire.

Similarly, reduction by compression and forward tension, as where a workpiece is pulled through dies or idle rolls, with no back tension, findsgreater applicability in the drafting of square or round sections thanin the reduction of flat sections, since these latter present a greatersurface for frictional contact with dies or rolls in comparison to thecross-sectional area of the stock to be reduced-a perfectly roundsection affording the optimum conditions of minimum frictional contactsurface to area of mass in this regard. Consequently, in flat sections,an excessive forward pull is needed to effect the draft,

resulting in the necking-down (in width) of flat stock beyond thecompressive zone.

Tensions back and forward may be applied to reduce flat stock through acompression means, but, unless such means is positively driven, as bylive rolls, to advance the material, the back tension must always belimited by the forward tension; otherwise, the work piece would movebackwards through the compression zone. Since the cross-sectional areaof the work piece after reduction is less than before reduction, thetotal forward tensile force applied, even if it were no more than equalto the back tensile force, would give substantially the same effect as ahigher tension per unit of sectional area. Add to this sufllcientadditional pull posteriorly of the die necessary to advance the stock,and the tension per unit of sectional area will have become so high asto exceed the elastic limit of the material, causing it to neck-down inwidth.

Opposed to this is the relatively heavy section of metal enteringthedie, to which is applied a tensile force less, than the forward pull,rendering the unittension fbackward) of such a low value asdrasticallymoilimit the drafting.

The reasons. forrthi'sllimitation are to be found in theprinciples-underlying reduction by compression alone: With fewexceptions, reduction by compression is limited to the production offlat metalstock, most of which is produced by of the several methods,tension alone is rarely used because of the difliculty experienced incontrollingthe radial components which cause the mass of a work piece todraw toward its draftaxis when 'tensibly stressed beyond its elasticlimit. This phenomenon, popularly referred to as necking-down, makesreductions of metal bodies 'by tension alone more feasible where rollingbetween driven'work rolls. -Heavy sections of stock are, preparatory torol1ing,.heated to such temperatures: wherein they are readilydeformable and are then forced into and through the work rolls whichreduce the gauge thickness and "effect the elongation thereof. '80 longas the metal is sufliciently thick to'provide a larger inner mass tobecome plastic under pressure,

and, thus, to extrude through the work rolls durrelatively no volume,the resultant reduction is rendered practically negligible beyond thispoint. This is true whether the material be hot or cold in the absenceof tension.

The high compression necessary to overcome the limit of elasticity ofthe metal is eifective to clamp the metal firmly between the rolls. Inthin gauge stocks, where extension of the inner mass is negligible, theclamping effect prevents the plastic fiow of metal so materially as topreclude reduction by compression alone beyond a limited extent. Withthe rolls in motion, this resistance to flow through the rolls resultsin a wringer action upon the work piece, causing a bulge of metal toaccumulate anteriorly of the pass, which in turn jams or wedges therolls, and creates enormous friction which is reflected in the excessivecompressive load factors beyond which a mill cannot go. This frictionalcomponent cannot be overcome by any known means in straight compressionrolling to allow reductions of net metal stock to gauge thicknesses lessthan .050" in widths up to 60' wide, nor less than .080" in widths of60" wide or more, this being true whether the stock is heated to hightemperatures, or is relatively cold.

In order to make possible the reduction of auge thickness of flat metalstock to less than .050" or .080", depending on the width, itisnecessary to subject the material to such a tensile stress upon theapproach side of the compression zone so as to materially lower theaverage unit compression stress at this point by reducing the bulge ofmetal that tends to lam into the pass. This tension is applied to thestock upon the entering side of the rolls, and is of such an order aswould cause the stock, after passing through the rolls to neck-down, ifit were applied forwardly of the latter. .This precludes the use of:stationary dies or idle rolls as the compressive means, and requiresthat the reduction be made through positively driven work rolls,

which not only compress the material, but effect the advancement thereofas well. I

Back tensions of the order contemplated under the present invention havenot been possible I heretofore in hot mill practice because of theextremely low elastic limit of flat stock under normal hot rollingconditions. Because of the low elastic limit. of hot roller stock, theproduction of relatively thin gauge fiat material, such as iron 'andsteel sheets and strip has not-been possible un er conventional hot millpractice. /Although the present invention is chiefly di rected to theapplication of tension in the reductiqn by rolling of metals having arelatively low limit of elasticity and is specifically concerned withthe application of back tension'to the hot rollingof steel strips;sheets and similar materials. it is also applicable to the reduction ofsuch metals or products in cold condition. Byrendering the tensioning ofsuch material anteriorly of the rolls possible, the limitation upongauge thicknesses producible by hot mills is entirely remved, renderingfeasible the reduction to gauges of .010" andless.

In addition to the work. factors involved, tension rolling is known tobe beneficial to the finish or surface condition of the metal in that itavoids excessive friction, reducing the probability of socalledpinchers, cobbles, etc., and insures that the metal tracks fairly intothe dies or rolls. Because of these improved operating conditions,substantial increases in speed have been made in cold reduction methods,increasing both the yield and quality of product produced on thepresentday mills. Furthermore, since a work piece is subjected to bothstatic and sliding friction in the roll pass, its metallic fibers aresubjected to 7 different degrees of stress, those acted upon by staticfriction becoming stressed differently than those acted upon by slidingfriction. Thus, this non-uniform stressing of the fibers naturallydistorts them, causing the overall metal piece to be unequallystrainedjand, consequently, wavy, distorted, and devoid of the much tobe desired flatness. Since tension reduces the frictional components, aspreviously noted, it is not the least of' its advantages that itminimizes the fiber distortion. and allows practically dead flatmaterial to be produced. Although pro'per lubrication can be consideredas an important'factor in reducing the troublesome friction, it cannotbe regarded as a substitute for tension in this respect, but rather asan adjunct'to the indispensable tension assisting toward the desiredend.

Because of the low limit ofelasticity of hot mill material, and becauseof the impracticability of applying eflicient lubrication thereto, ithas not been possible heretofore to enjoy any of these advantagesaccruing to cold reduction methods, whereby hot rolling has had toterminate at the heavier gauges. As has been said, any conslderabletension either before or after the introduction of the hot metal to therolls, causes it to neck-down in lateral dimensions, whereby, undercontinuous mill operations, the finishing material is prohibitivelynarrowed in relation to its starting width.

By way of distinction, there'has been, in the past, means for applyingvery l ght tensions intermediatethe stands of continuous hot mills,which, by so-called looping" devices serve to make possibleasufllciently light tension between the stands, by providing manualcontrol of the delivery speeds of the mill motors. These. however, arelittle more than slack or loop prevention devices, and, as such. are oflittle value in reducing the area of rolling contact, or the compressiveloads of the mill, even though they greatly assist in securing thecorrect tracking of the strip, thus to minimize edge-curling, cobbles,pinchers, lappers, etc. The capabilities of the hot mill have thus beendetermined largely by the elastic limit of the material and its abilityto withstand very slight tensions without necking-down in width.

The plienomenonofnecking-down, or that tendency of metals undersufllcient tensile stress to flow plastically to gather toward, andevenly distribute its mass about, the axis of draft, is directlycorrelated to the elongation per unit length of specimen underconsideration. The longer the unit length of specimen under stress, thegreater is the elongation, and proportionally greater is thenecking-down. Conversely, the shorter the unit length of specimen understress, the less the elongation, and the less the extent ofneckins-down, even though the tensile stress in either caseremains thesame.

without incurring the risk of appreciable necking-down,

In hot mill practices, particularly in tandem hot strip mills, thedistance. of eighteen (18) feet or more between passes is such as torender the application of sufficient tension appreciably to alleviatethe frictional component of the succeeding compressive zone, or rollpass, impossible without necking-down the strip an inadmissible amount.7 Therefore, although very moderate tensions have been applied to hotstrip mills by the manual control of the mill motors with the assistanceof so-called looping devices, thus to reduce lapping, pinchers, cobbles,etc., no substantial diminution of the rolling frictional components' orthe compressive load factors have been possible, and the gauge valueshave been accordingly limited to the heavier thicknesses of strip.

In recognizing that the amount of neckingdown in width of materials oflow elastic limit is dependent, among other things, upon the lengthsubject to tension, which would normally correspond to the distancebetween the passes of a continuous mill, I have discovered that, bydiminishing the tensioned length of material to a dimension which is assmall as-possible, an exceedingly high tension can be applied from whichis derived all of the benefits attendant upon the application of tensionto cold rolling, as well as other advantages which will be discussedmore in detail hereinafter, without necking-down the material more thanan allowable amount In the preferred embodiment of my invention, Ipropose to apply a tensile force to the material upon the approach sideof each reducing stand, which, in point of application, is as close aspracticable to the work rolls. As an illustration, the tensioned lengthin proportion to the width of the material, may be unity orless. Ingiving effect to this teaching in amanner which would appear to be thebest embodiment, it is contemplated that a set of auxiliary rolls beapplied to the strip upon the approach side of the mill as close aspossible to the workv rolls, which auxiliary rolls will be sufilcientlycompressively loaded to effect some reduction in thickness of the stripand be so controlled, in relation to the preceding set of work rolls,and to the succeeding set of work rolls, that the hot strip upon theentry side thereof will be under substantially no tension, but

will be under relatively high tension from the exit side thereof to thereducing zone of the succeeding work rolls to which they are proximate.Such an apparatus will function thus to subject the material, as itenters the work rolls, to a very high order of back tension over suchminimum distance of strip length that, when posed against the lateralretaining effect derived from the clamping action of the auxiliary rollsand the work rolls upon the strip, causes a minimum amount ofnecking-down. Such neckingdown may be of an order in which, on thethicker sections the lateral extrusions or flow tending to widen thestrip in the compressive zones will offset that lost by necking-down inresponse to tively little compressive function to bring the 1 work pieceto gauge, and thus act more as sizing and shaping devices than asdeforming dies. The only limits upon this seem to be that the materialshall not be reduced in thickness by stretching beyond the point wheretractive effort be tween the rolls and work piece ceases. Because theefiect of such auxiliary rolls is to regulate the amount of material fed.into the work rolls, they have been designated as meteringrolls, bywhich term, for convenience and clarity of expression, they will behereinafter called.

It is, therefore, the primary object of the present invention to providenovel method and means for reducing materials capable of plastic.deformation, and particularly such materials havcompressive and tensileforces thereon in which such tensile forces are'of an order sufllcientto exceed the elastic limit of such material, without necking-down thematerial in width more than an admissible amount.

It is another object to provide means for hot rolling strip steel undertension wherein the tension applied is in excess of the elastic limit ofthe material, while critically controlling the necking-down thereof inwidth as well as in thickness.

It is still another object to provide means for tension-rollingmaterials of low elastic limit which will enable the adoption and use ofmuch lighter equipment than is at present possible, which will alsopermit of the substantial reduction iii-number of passes to the samegauge, and to lighter gauges, in the case of hot strip steel.

It is a further object to provide for the production of hot strip steelto gauges heretofore obtainable only by cold mill methods.

It is a further object to provide for the production of a strip steel insuch a manner that, from the same slab temperatures as now applied,fewer reductions will be necessary to attain an exceedingly light gauge,thus preserving the heat of the work piece, and permitting it to becoiled at very high temperatures later effective in self-annealing thecoil.

' It is a further object to provide for the production of hot strip insuch a manner that its surface will be relatively free from scale andother imperfections, 1 and which, in gauge and finish, will closelyapproach cold strip produced by cold reduction methods.

' It is a further object to increase the yield of hot strip mills peroperating periodind to minimize the production of scrap partici larly onwide width sheets and strip.

It is a further object sufficiently to stretch hot strip materialwithout undue necking-down to \elongate fibers of the metal sufficientlyso as to render subsequent cold reduction and/or roller 2-high millswhere 4-high mills are now em- '4 'ployed, and with better uniformity ingauge of -Many other objects and product across'the width.

advantages, particularly with respect to the economics oi present daynot mill operations, and hot mill operations versus cold milloperations, will become apparent from the following specification whenconsidered in conjunction with the accompanying drawings, in which:

Figure 1 represents a schematic elevational view of a continuous mill towhich apreierred form of the invention has been applied.

Figure 2 is a partial plan view of Figure 1, illustrated with thebacking-up rolls removed from the 4-hlgh rolls.

Figure 3 is a. schematic plan'view illustrating the phenomenon ofnecking-down as would result from the application of substantialtensions to present hot mill equipment and practices, and

Figure 4 is a diagrammatic view illustrating an electric control iorsucha mill designed to give eil'ect to the present invention.

In the drawings, with reference to Figures 1 and 2, a continuoushot millcomprising conventional -high stands l-l, having work rolls 2, andconventional looper control 3 between stands,

is provided with metering rolls 4 at the approach;

side of each oi. the d-hig h mill stands, and] as close thereto as ispracticable. Guides 5 may be applied todacilitate the passage of thematerial between the metering rolls 4 and the work rolls- 2, in eachinstance. A coiler 6, adapted'to exert a moderately low tension upon thematerial as it is delivered from the last finishing stand, is shown forcoiling the material in even, tight coils. It is obvious that, if it isdesirable, the flat type of run-out tables may be provided for use withthe present invention as is customary on the hot mills currently invogue.

The arrangement of parts issue?! that a hotstrip S, between its point ofexit from work rolls 2 of one stand, to its point of entry into themetering rolls 4 of the succeeding stand, is under no tension, ortension or a moderately low value, such as is conventionally applied.This is insufllcient to cause necking-down, but assists in the correctfeeding of the material into the metering rolls, and prevents lappers,pinchers, and cobbles at th s point. Thus, throughout the larger linearextent of strip disposed between the work passes,

no tension greatly in excess of the elastic limitof the work piece isapplied. For this purpose. and as will be later described in connectionwith Figure 4, the metering rolls 4 are driven in relat on to thepreceding work rolls 2 so as to receivethe material at substantially thesame rate as it is delivered by the latter. To insure that thisrelationship is maintained, the convenional looper or tensioning rollcontrol 3 may be provided. The work rolls 2 succeeding the meteringrolls- 4, with which the latter are associated, are con trolled toforward the strip at a' greater rate than it is delivered by themetering rolls, thus to exceed the elastic limit of the material over avery short distance of its length, so as to give eiIect to the backtension, and its attendant advantages, as previously described. Tooppose this tension, the-metering rolls clamp the strip and!- cientlyfirmly to perform a slight reduction of the material-{rem one to tenpercent reduction in gauge being representative.

In the interest of winding moderately tight coils, the coiler 6 may becontrolled to exert a moderately low tension on the strip as it leavesthe last finish stand to be able to wind -suitable coils withoutnecking-down, and without allowing substantial oxidation of the innerfaces thereof. In this connection, the scale which is formed on thesteel at high temperatures is known to be deoxidized or reduced back tosteel again when virtue of the heavy back tension applied, would causeany scale that is formed to be, cracked oil and blown away upon theapproach side of each work zone, thus leaving only slight amounts ofsecondary scale, which is reformed on the strip between passes, andbetween the last pass and the coiler. This minimization of scaling, andthe partial reduction of what scale does form, eflects anadvantageoussavings in pickling practices.

In Figure 3 there is disclosed a conventional type of mill in which thestrip is passed between work rolls 2-2 in the directionof the arrow A.When the mill is thus threaded but not running, and tension is applied,under this static condition the strip tends to reduce in width, comingto its narrowest dimension at a point equidistant from the compressivezones of the work rolls 1-2, as is represented by the broken lines inthis figure. The material at such zones tends to extrude sideways,provided it is of suiflcient thickness to do so, as is indicated at I inFigure 3, and is afiorded lateral support by the clamping action of therolls. When the mill is operating with such tension, the effect is tocause the necking-down action to progress between stands (the laterallyclamped strip becoming tractable under motion) so that the'narrowestpoint occurs immediately adjacent the entry of the material into thesecond stand, as is represented by the solid lines 8. If appliedthroughout a suflicient number of passes, the strip would ultimatelylose most of its transverse dimension and thus become unsuitable forwide strip applications.

As contrasted with this, the action of the present invention isdisclosed in Figure 2 in which the strip S, between the work rolls 2 andthe metering rolls 4a and moving in the direction of the arrow A, 'isnot under such tension as will cause substantial necking-down, whilebetween the metering rolls 4a and the succeeding work rolls 2a. thematerial, though subjected to heavy tension, is of insuiflcientlongitudinal extent to cause the necking-down factor to be appreciable.

In thisflgure it will be seen that the strip S, moving in the directionof the arrow A, extends between the work rolls 2 and the metering rolls4a at maximum width since, in this and corresponding zones, it is undersubstantially notension. Between the metering rolls 4a and the workrolls in a heavy tension in excess of the elastic limit is imposed uponthe strip, which is caused to neck down, as indicated at N. .Since thisis not a static load, but is applied with the mill's running to advancethe; material, the narrowest portion of the strip, as a result ofnecking down, occurs immediately before the compressive zoneof the workrolls 20. Upon entering this zone, the material, if suflicientlythick todo so, spreads laterally asis indicated at 1:1, whereby a substantialamount, it not all, of its pre-tensioned, width may be recovered. Fromthe work rolls 2a' to the metering rolls (not shown) of the succeedingpass, the strip is again relieved from substantial tension and tends tomaintain the maximum width resulting from its compression in the workrolls In, as is'indicated at la. The

same procedure is repeated in connection with the succeeding andpreceding metering rolls and work rolls (if any) so that the material isrepeatedly necked down, as indicated at N, and restored to width as atIa, and forwarded under a substantial absence of tension, whereby itsrecovered width is substantially retained. In this manner, even shouldthere be sustained some over-all diminution in strip width by virtue ofthe necking down, it will be of such a moderate amount as to beallowable for in advance by selecting a work piece suflicientlyover-size to produce a final strip of the required width under theconditions of properly regulated tension here contemplated.

' Variou methods may be utilized to provide control of the amount oftension developed in the strip between the work rolls of a mill standand the associated metering rolls. The. arrangements to this end hereindescribed are suitable for giving effect to the invention in asatisfactory manner, but should be regarded as illustrative rather thanas restrictive.

I Electrical control might be employed, for example,- in an arrangementsimilar to that shown in Figure 4. In this system, tension control isobtained by using variable field strength applied to the shunt fields ofthe work roll drive motor and to the similar field of the associatedmetering roll motor.

There is illustrated in Figure 4 aportion of the finishing stands of atandem rolling mill. The operating speed or the mill as a unit iscontrolled by the variable voltage method of the Ward- Leonard system inth illustrated embodiment. One or more constant speed main generator Ghave their shunt fields l2 separately excited by the exciter E, thevalue of excitation and, consequently, the terminal voltage of the maingenerator being variable by operation of the motor driven rheostat H. Apreselected variable voltage determined by the desired operating speedor the mill is impressed upon the various mill drive motors through thevariable voltage bus lines 9 and 9'.

'The mill drive motors W1, We (the armatures of which are mechanicallyconnected to the work rolls 2) have their, shunt fields l3 excited froma constant potential source of current, supplied through the lines l4and M, from any suitable source of supply, as for instance, a generalexciter unit 0. To provide control for changing the operating speed ofan individual mill stand. a counter-E. M. F. exciter L is placed inseries with the shunt field l3 of each individual mill stand drive motorW, the speed of which is inversely proportional to it shunt fieldstrength. The excitation of the shunt field 23 of the exciter L iscontrolled by means of a manually operated rheostat II in series withthis field.

A pair of metering rolls 4, disposed closely adjacent to the work rolls2 at the entry side of the latter, are mechanically connected to a motorM2, the shunt field it of which is excited from the common supply linesI4 and Hi. This field I6 is also in series with the exciter L and thmotor speed is variable in response to operation of the rheostat l1.Thus, the speed or the drive and tension combination of any stand may bechanged as a unit without disturbing the relation therebetween, andwhile maintaining the drive and tension combinations of other stands ata constant speed. It is to this rheostat II that the iooper device 3 maybe mechanically conneoted automatically to control the condition oftension or slack between adjacent stands (e. g., A and B, Fig. 4),

Independent speed control of the metering rolls 4 is obtained by manualoperation of the rheostat l9 which is in series only with the shuntfield I6 of the metering roll motor, of which M2 is typical.

Provisions is made for controlling th amount of tension in that portionof the strip which is between the work rolls 2 and the associatedmetering rolls 4 by the use of the manually operated rheostat I8- whichis also in series with the shunt field iii of the motor M2. Decreasingthe resistance in the rheostat l8 increases the field strength of themotor M2 resulting'in a decrease in speed of this machine.

For the purpose of describing the operating sequence, let it be assumedthat the mill operator decides upon the drafting practice to be followedin reducing the work piece from its roughing stand exitgauge to thegauge desired in the finished product. The per cent reduction to whicheach stand subjects the work piece includes consideration of therequired screwdown pressure, linear speed or strip travel anddistribution of load among the individual stands within proper limits orthe respective mill stand drive motor ratings.

The finishing train (of which the stands illustrated in Figure 4 may beconsidered a part) running light, is accelerated to desired rollingspeed by the progressive increase of voltage generated in the maingenerators G by operation of the motor driven rheostat I i Asconventional equipment, each stand. is provided with a speed measuringand indicating device such as, for example, a tachometer generator withassociated meters (not shown) calibrated in linear speed of strip traveland having adjustment of calibration for difierent roll diameters. Eachset of metering rolls should also be provided with similar devices. Theoperator observes the indicated peripheral speeds of the work rolls 2and the associated tension rolls 4, and, if the speed of the latterdiiiers from that of the former, the rheostat 19 leadjusted so that thesurface speed or th metering rolls 4 is matched with that of theassociated work rolls 2. This regulation is particularly advantageouswhen work rolls of a certain diameter are replaced by others having adifferent diameter.

Assuming that the front end of the work piece S has passed through thefirst finishing stand A and has been subjected to a Dre-Selectedreduction, it is conveyed forward until it is engaged by the meteringrolls 4 of stand B. The horizontal component of the compressive forceexerted by the tension rolls 4 continues to move the work piece intoengagement with th work rolls 2 of stand B in which it is subjected toan additional pre-selected reduction.

When reduction is initiated in stand 3, the drive motor W: of this'stand draws an increased value of load current, causing a substantialvoltage drop across its compensating field-winding 20. a load relay 2|to operate contactorlz shunting out a pre-selected portion 24 of theresistance in the tension control rheostat l8 suflicient to result inthe relative reversal of the torque exerted by the motor M2. Thisrelative torque reversal in the motor M: creates a generator actionwhich exerts a retarding infiuence'or drag upon the forward movement orthe strip S through the metering rolls 4, causing tension to bedeveloped This drop is utilized .to energize the coil of in' the stripbetweenthese rolls and the work rolls 2 with which the former areassociated.

The amount of tension developed in the strip S is proportional to thecrease in the shunt field (l6) strength of the/inotor Mzcaused byshunting out some portion 24 of the resistance in rheostat l8. It willbe recalled that for a given design of motor, the torque is proportionalto the product of the field flux and the armature current. When thedirection of the armature current is reversed, the product becomesnega-.

tive and the torque is reversed. The speed of the motor varies inverselyas the flux or ampere turns in the shunt field circuit.

When the resistance, of rheostat I8 is shunted out the normal speed ofthe motor Ma tends to decrease a proportional amount.- Since, however,this motor is mechanically connected to the metering rolls l which areturned by the strip S as the latter is advanced by the work rolls 2, the

. armature of the metering roll drive is made to revolve at the greaterpreviously established speed, thus creating counter E. M. F. greaterthan the applied voltage. Accordingly, the armature current flows in theopposite direction, and a reverse torque is developed in the machine M2,causing it to become, in effect, a drag generator.

It will be understood that the foregoing, illustrated in conjunctionwith but two stands (A and B) of the finishing train, involving workroll motors W1 and W2, and metering roll motor-generator M2, isapplicable to as many stands of the finishing and even roughing trainsor reversing mills as it is desired to apply the invention.

By virtue of the heavy tension thus realized, the wave or bulge of metalwhich is known to form at the approach side of conventional hot mills isentirely eliminated, substantially reducing the friction and area ofcontact between the work rolls and the strip, thus resulting in asubstantial minimization of the compressive load to which the mill mustsubject the strip for the same amount of reduction, or resulting in agreater reduction at the same compressive load.

Among the outstanding advantages of the present. method is that whichwill enable hot strip steel to be reduced to gauges heretoforeimpossible of attainment; Thus, it is entirely feasible in accordancewith the present invention to reduce strip steel down to tin plate gaugeand then, by virtue of the residual heat of the material in coiled form,to enjoy the benefits of self-annealing, for strain removal and graingrowth, to produce a material without any further processin that willcompete directly with cold strip. In this respect, it will be borne inmind that coldreduced strip must be annealed one or more times, andsubjected to several cleaning operations as a'result thereof. Thepresent invention, to the contrary, entails no annealing, and has onlyvery light pickling requirements, as previously described. Furthermore,from the standpoint of physical properties, the hot strip herecontemplated need not be roller or stretcher leveled to meet theexacting requirements of the present cold strip trade, and might needonly one light pass on a-cold mill to develop the surface properties nowenjoyed by material produced predominately by cold mill methods.

- Because ofthe great relief in compressive stress, hot mills forrolling metal hot from slab to very thin gauge strip are envisioned thatrequire no more than four or five stands, including both the roughingand finishing trains, where now ten are employed. In fact, by properappliwidth is not objectionable.

cation of the metering function herein contemplated, at large mass ofmetal, such as an ingot, bloom, billet, or slab, if heated nearly to themelting point. could be reduced to sheet or stripgauge in one pass bycorrelating the metering roll speed to the work roll speed to stretchthe work piece to gauge, or nearly to gauge, before entering the workrolls. Such an application could advantageousLy be made in connectionwith continuous casting processes, immediately after the metal haspassed from the molten to the solid state.

In addition, the more conventional types of hot mills may not need to beof the same heavy construction as at present, and, in fact, 2-high millsinstead of the present 4-high mills ar believed to be entirely feasiblewithin the practice of the present invention. Thus are the economics ofhot mill practices vastly improved, and rendered superior to combinedhot and cold mill methods, which have heretofore been necessary in theproduction of light gauge sheet and strip. It will be appreciated thatthese economies are more menifest on wide gauge material, wherein theactual pressure loads, and the difllculty of feeding sheets throughwithout edge stretching, cobbles, pinchers,-and so forth, are greatlyenhanced, although the invention will find many economical applicationsto the narrow strip field, as well as in the reduction of hot rods andwire, or any material, whether hot or cold, having low elastic limit.Nor need the work piece be of substantial longitudinal dimension toenjoy the full benefits of this invention, since the advantages flowingfrom back tension may be applied to any work piece sufiiciently long toextend throughout the major portion of its length during the reductionfrom the metering rolls to the work rolls at any given pass.

Even though the invention has been disclosed as applied to continuousmills, it will be appreciated that it is equally applicable to reversingmills, in which case a pair of metering rolls will be disposed at eachside of the work rolls with those upon the approach side of the millbeing used for tensioning purposes, while those on the exit sidecan bedisposed so as to assist in the forwarding of the strip through themill, or may be lifted out of the pass line so as to permit the strip topass therebetween without engagement. I

It will be equally obvious that other tensioning means besides tensionrolls as here illustrated can applied without departing from theteachings of this invention. Various types of frictional drags, fromdynamic brakes to staggered bearings (as in breaker rolls to impartsinuous travel) may be utilized, so long as the distance through whichthe tensioned work piece must run without lateral support is minimized,and fairly well proportioned to the width of piece.

The advantages to. be derived from placing the metering rolls as closeto the. work rolls as possible can well be appreciated from theforegoing specification. However, the metering function can be perfomedjust as satisfactorily by rolls placed any distance from the work rolls,or at any point intermediate the stands of a tandem mill, if more. orless necking-down in In fact, a controlled necking-down can be derivedby such an arrangement that can be turned to advantage rather thanotherwise, while the prestretchingto-gauge benefits of unloading themill work rolls '75 to reduce friction and produce stress-free, fiatcussed closed-roll arrangement.

From the foregoing it will be seen that the present invention isconcerned essentially with the prestretching of metal in a controlledmanner nearly to gauge as it enters work rolls, to relieve the latter ofexcessive frictional and compressive load factors, using the work rollsthemselves as a tractive clamp against which tension is applied as themetal is forwarded thereby. In hot mill applications, such as continuoushot strip mills, the length of metal subjected to stretching tension ispreferably minimized anteriorly of the work rolls, which clamp the metal.to maintain its dimension of width, whereby the ratio of tensionedlength to supported width is as low as possible to reduce thenecking-down in width. Therefore, stated another way, the invention isessentially a means and method of controlling the necking-down of metalsections, in both transverse dimensions (thickness and width) to give afinished work piece of the required shape and uniformity.

Since the apparatus for giving effect to the novel relationships hereindisclosed may assume a variety of forms, it is intended that the hereindisclosed embodiment be regarded as illustrative rather thanrestrictive. Accordingly, it is not intended that the present inventionbe limited thereby, other than is called for by the recitation of theappended claims.

I claim:

1. In the production of flat metal objects of low elastic limit byrolling, such as hot steel sheets and strip, the improvement whichincludes passing such an object through reducing rolls driven to advancethe work piece, resisting the advance of the work piece into the rollsby a force applied uniformly thereto closely adjacent its point of entryinto said rolls, said force being inexcess of the elastic limit of thematerial.

2. The method for continuously reducing fiat metal shapes of low elasticlimit which includes successively passing such material through aplurality of reducing zones spaced apart a distance equal to a multipleof the width of the material, maintaining the greater part of saidmaterial between zones under insufiicient tension to cause appreciablenecking-down thereof, while subjecting a small part of said materialbetween zones, adjacent the entry of each, to a tension sufficient tostretch the material.

3. The method of reducing metals having a relatively low limit ofelasticity while undergoing reduction, which includes forwarding themetal through reducing means to reduce its gauge thickness, applying atensile pull to the metal in excess of its elastic limit so as to resistits entry into the reducing means, limiting the necking-down in widththereof by supporting the extremities of the tensioned portion of themetal to counter the necking-down forces effective to reduce the widthgauge thereof, and maintaining said supported extremities in closeproximity.

4. The method of reducing flat metal stock which includes stretchingthe'stock between two bearings spaced less than the width of the stockapart, and compressively rolling said stock by at least one of saidbearings.

5. The method, of reducing fiat metal stock which includes stretchingthe stock between two bearings spaced less than the width of the stockapart and compressively rolling said stock by at least one of saidbearings, while retaining substantially the prestretched width of the.stock at said bearings.

7. The method of continuously reducing successive portions of fiat metalstock which includes stretching the stock longitudinally between twobearings spaced along its length not more than the width of the stockapart, moving the stock between said bearings, and continuouslyretaining the'greater transverse dimension of the stock at said bearingsduring the stretching and moving operation.

8. The method of continuously reducing successive portions of fiat metalstock which includes stretching the stock longitudinally by applyingtension thereto in excess of its elastic limit, supporting the tensionedportions of said stock to preserve the flatness and with thereof at aplurality of bearings spaced along the stock's length not more than thewidth of the stock apart, and continuously forwarding the stock.

9. The method of reducing flat metal stock which includes advancing thestock through and by driven compression rolls to reduce its thickness,tensioning the stock upon the approach side of the rolls for a distancealong its length not in excess of the width of the stock, therebysimultaneously stretching and compressing the stock progressively alongits length, and supporting said stock at least adjacent the extremitiesof the tensioned extent thereof to retain substantially its prestretchedwidth during the reducing operation.

10. The method of reducing flat metal stock continuously which includestensioning and compressing the stock simultaneously to exceed itselastic limit, and supporting the stock throughout the tensionedportion. thereof by width-retaining bearings spaced longitudinally ofthe stock no more than the width of the stock apart, at least one ofwhich acts on said stock to effect the compressing thereof.

11. In a hot rolling of metals the improvement which includes passing apreheated body between a pair of compression rolls to reduce andelongate said body, introducing said reduced and elongated body into asecond pair of compression rolls, and, within a distance from the secondpair of rolls not substantially greater, and preferably less, than thewidth of said body, introducing said body into a third pair ofcompression rolls, driving the first and third pairs of rolls to forwardthe body, driving the second pair of rolls at a surface speed so relatedto the surface speed of the first pair of rolls as to minimizetensioning the body therebetween, and driving the third pair of rolls ata higher surface speed than said second pairs of rolls, while the bodyis in engagement with the three pairs of rolls.

12. Apparatus of the class described comprising a plurality of principalwork rolls, a plurality of metering rolls interposed between said workrolls, said metering rolls being placed closely adjacent the latter uponthe entering side thereof, means for driving proximate metering and workrolls difierentially, and means for .8 aeaaeo'a work piece at points nogreater a distance apart than the maximum width of work piece that canbe accommodated thereby, said means being adapted to stretch the workpiece therebetween.

\ and to'support its edges to maximum width position during thestretching operation.-

EDWIN T. LORIG.

